U.S. patent application number 15/495089 was filed with the patent office on 2017-08-10 for thermoset nanocomposite particles, processing for their production, and their use in oil and natural gas drilling applications.
This patent application is currently assigned to Sun Drilling Products Corporation. The applicant listed for this patent is Sun Drilling Products Corporation. Invention is credited to Jozef BICERANO.
Application Number | 20170226397 15/495089 |
Document ID | / |
Family ID | 36615570 |
Filed Date | 2017-08-10 |
United States Patent
Application |
20170226397 |
Kind Code |
A1 |
BICERANO; Jozef |
August 10, 2017 |
THERMOSET NANOCOMPOSITE PARTICLES, PROCESSING FOR THEIR PRODUCTION,
AND THEIR USE IN OIL AND NATURAL GAS DRILLING APPLICATIONS
Abstract
Use of two different methods, either each by itself or in
combination, to enhance the stiffness, strength, maximum possible
use temperature, and environmental resistance of thermoset polymer
particles is disclosed. One method is the application of
post-polymerization process steps (and especially heat treatment)
to advance the curing reaction and to thus obtain a more densely
crosslinked polymer network. The other method is the incorporation
of nanofillers, resulting in a heterogeneous "nanocomposite"
morphology. Nanofiller incorporation and post-polymerization heat
treatment can also be combined to obtain the benefits of both
methods simultaneously. The present invention relates to the
development of thermoset nanocomposite particles. Optional further
improvement of the heat resistance and environmental resistance of
said particles via post-polymerization heat treatment: processes
for the manufacture of said particles; and use of said particles in
the construction, drilling, completion and/or fracture stimulation
of oil and natural gas wells are described.
Inventors: |
BICERANO; Jozef; (Midland,
MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Sun Drilling Products Corporation |
Belle Chasse |
LA |
US |
|
|
Assignee: |
Sun Drilling Products
Corporation
Belle Chasse
LA
|
Family ID: |
36615570 |
Appl. No.: |
15/495089 |
Filed: |
April 24, 2017 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
15294089 |
Oct 14, 2016 |
9630881 |
|
|
15495089 |
|
|
|
|
15165136 |
May 26, 2016 |
9505974 |
|
|
15294089 |
|
|
|
|
15008473 |
Jan 28, 2016 |
9394471 |
|
|
15165136 |
|
|
|
|
14829253 |
Aug 18, 2015 |
9267071 |
|
|
15008473 |
|
|
|
|
14495454 |
Sep 24, 2014 |
9175209 |
|
|
14829253 |
|
|
|
|
13717636 |
Dec 17, 2012 |
9359546 |
|
|
14495454 |
|
|
|
|
13629018 |
Sep 27, 2012 |
8466093 |
|
|
13717636 |
|
|
|
|
13353542 |
Jan 19, 2012 |
8278373 |
|
|
13629018 |
|
|
|
|
13340080 |
Dec 29, 2011 |
8455403 |
|
|
13353542 |
|
|
|
|
12980510 |
Dec 29, 2010 |
8088718 |
|
|
13340080 |
|
|
|
|
12870076 |
Aug 27, 2010 |
7902125 |
|
|
12980510 |
|
|
|
|
11323031 |
Dec 30, 2005 |
7803740 |
|
|
12870076 |
|
|
|
|
60640965 |
Dec 30, 2004 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C04B 14/022 20130101;
C09K 8/80 20130101; C09K 8/32 20130101; E21B 21/062 20130101; C09K
8/24 20130101; C09K 8/64 20130101; C09K 8/52 20130101; C09K 8/62
20130101; C04B 16/082 20130101; C09K 8/42 20130101; C09K 8/035
20130101; E21B 43/04 20130101; C09K 8/473 20130101; Y10T 428/2982
20150115; C09K 8/725 20130101; E21B 43/267 20130101; C04B 24/2688
20130101; C09K 2208/34 20130101; C04B 16/082 20130101; Y10T
428/2998 20150115; B82Y 30/00 20130101; C09K 2208/10 20130101; C09K
2208/28 20130101; E21B 43/26 20130101; E21B 33/13 20130101; C04B
20/008 20130101; C09K 8/805 20130101 |
International
Class: |
C09K 8/035 20060101
C09K008/035; C09K 8/473 20060101 C09K008/473; E21B 21/06 20060101
E21B021/06; C09K 8/80 20060101 C09K008/80; C04B 24/26 20060101
C04B024/26; C04B 14/02 20060101 C04B014/02; C09K 8/24 20060101
C09K008/24; C09K 8/62 20060101 C09K008/62 |
Claims
1.-89. (canceled)
90. A method for treating a well penetrating a subterranean
formation comprising: (a) mixing into a drilling mud formulation an
effective amount of a polymeric nanocomposite spherical bead
comprising: a polymer matrix; and from 0.001 to 60 volume percent
of nanofiller particles possessing a length that is less than 0.5
microns in at least one principal axis direction; said nanofiller
particles comprising at least one of fine particulate material,
fibrous material, discoidal material, or a combination of such
materials, said nanofiller particles being selected from the group
consisting of natural nanoclays, synthetic nanoclays or mixtures
thereof wherein said nanofiller particles are substantially
dispersed throughout said polymeric nanocomposite spherical bead,
wherein said polymeric nanocomposite spherical bead has a diameter
ranging from 0.1 mm to 4 mm; and (b) introducing said drilling mud
formulation with said effective amount of the polymeric
nanocomposite spherical bead into said well.
91. The method of claim 90, wherein said nanofiller particles
possess a length that is less than 0.5 microns in at least one
principal axis direction and an amount from 0.1% to 15% of said
polymeric nanocomposite spherical bead by volume.
92. The method of claim 90, wherein said polymer matrix comprises
at least one of a thermoset epoxy, a thermoset epoxy vinyl ester, a
thermoset polyester, a thermoset phenolic, a thermoset
polyurethane, a thermoset polyurea, a thermoset polyimide, or
mixtures thereof.
93. The method of claim 90, wherein said polymer matrix comprises a
terpolymer.
94. The method of claim 93, wherein said polymer matrix is a
styrene-ethylvinylbenzene-divinylbenzene terpolymer.
95. The method of claim 90, wherein said nanofiller is natural
nanoclays.
96. The method of claim 90, wherein said nanofiller is synthetic
nanoclays.
97. The method of claim 90, wherein said nanofiller is a mixture of
natural and synthetic nanoclays.
98. The method of claim 90, wherein said polymeric nanocomposite
particle is blended with other solid particles including at least
one of sand, resin-coated sand, ceramic, and resin-coated ceramic.
Description
APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/640,965 filed Dec. 30, 2004.
FIELD OF THE INVENTION
[0002] The present invention relates to lightweight thermoset
polymer nanocomposite particles, to processes for the manufacture
of such particles, and to applications of such particles. The
particles of the invention contain one or optionally more than one
type of nanofiller that is intimately embedded in the polymer
matrix. It is possible to use a wide range of thermoset polymers
and nanofillers as the main constituents of the particles of the
invention, and to produce said particles by means of a wide range
of fabrication techniques. Without reducing the generality of the
invention, in its currently preferred embodiments, the thermoset
matrix consists of a terpolymer of styrene, ethyvinylbenzene and
divinylbenzene; particulate carbon black of nanoscale dimensions is
used as the nanofiller, suspension polymerization is performed in
the presence of the nanofiller, and optionally post-polymerization
heat treatment is performed with the particles still in the reactor
fluid that remains after the suspension polymerization to further
advance the curing of the matrix polymer. When executed in the
manner taught by this patent, many properties of both the
individual particles and packings of said particles can be improved
by the practice of the invention. The particles exhibit enhanced
stiffness, strength, heat resistance, and resistance to aggressive
environments; as well as the improved retention of high
conductivity of liquids and gases through packings of said
particles in aggressive environments under high compressive loads
at elevated temperatures. The thermoset polymer nanocomposite
particles of the invention can be used in many applications. These
applications include, but are not limited to, the construction,
drilling, completion and/or fracture stimulation of oil and natural
gas wells; for example, as a proppant partial monolayer, a proppant
pack, an integral component of a gravel pack completion, a ball
bearing, a solid lubricant, a drilling mud constituent, and/or a
cement additive.
BACKGROUND
[0003] The background of the invention can be described most
clearly, and hence the invention can be taught most effectively, by
subdividing this section in three subsections. The first subsection
will provide some general background regarding the role of
crosslinked (and especially stiff and strong thermoset) particles
in the field of the invention. The second subsection will describe
the prior art that has been taught in the patent literature. The
third subsection will provide additional relevant background
information selected from the vast scientific literature on polymer
and composite materials science and chemistry, to further
facilitate the teaching of the invention.
A. General Background
[0004] Crosslinked polymer (and especially stiff and strong
thermoset) particles are used in many applications requiring high
stiffness, high mechanical strength, high temperature resistance,
and/or high resistance to aggressive environments. Crosslinked
polymer particles can be prepared by reacting monomers or oligomers
possessing three or more reactive chemical functionalities, as well
as by reacting mixtures of monomers and/or oligomers at least one
ingredient of which possesses three or more reactive chemical
functionalities.
[0005] The intrinsic advantages of crosslinked polymer particles
over polymer particles lacking a network consisting of covalent
chemical bonds in such applications become especially obvious if an
acceptable level of performance must be maintained for a prolonged
period (such as many years, or in some applications even several
decades) under the combined effects of mechanical deformation,
heat, and/or severe environmental insults. For example, many
high-performance thermoplastic polymers, which have excellent
mechanical properties and which are hence used successfully under a
variety of conditions, are unsuitable for applications where they
must maintain their good mechanical properties for many years in
the presence of heat and/or chemicals, because they consist of
assemblies of individual polymer chains. Over time, the deformation
of such assemblies of individual polymer chains at an elevated
temperature can cause unacceptable amounts of creep, and
furthermore solvents and/or aggressive chemicals present in the
environment can gradually diffuse into them and degrade their
performance severely (and in some cases even dissolve them). By
contrast, the presence of a well-formed continuous network of
covalent bonds restrains the molecules, thus helping retain an
acceptable level of performance under severe use conditions over a
much longer time period.
[0006] Oil and natural gas well construction activities, including
drilling, completion and stimulation applications (such as
proppants, gravel pack components, ball bearings, solid lubricants,
drilling mud constituents, and/or cement additives), require the
use of particulate materials, in most instances preferably of as
nearly spherical a shape as possible. These (preferably
substantially spherical) particles must generally be made from
materials that have excellent mechanical properties. The mechanical
properties of greatest interest in most such applications are
stiffness (resistance to deformation) and strength under
compressive loads, combined with sufficient "toughness" to avoid
the brittle fracture of the particles into small pieces commonly
known as "fines". In addition, the particles must have excellent
heat resistance in order to be able to withstand the combination of
high compressive load and high temperature that normally becomes
increasingly more severe as one drills deeper. In other words,
particles that are intended for use deeper in a well must be able
to withstand not only the higher overburden load resulting from the
greater depth, but also the higher temperature that accompanies
that higher overburden load as a result of the nature of geothermal
gradients. Finally, these materials must be able to withstand the
effects of the severe environmental insults (resulting from the
presence of a variety of hydrocarbon and possibly solvent molecules
as well as water, at simultaneously elevated temperatures and
compressive loads) that the particles will encounter deep in an oil
or natural gas well. The need for relatively lightweight high
performance materials for use in these particulate components in
applications related to the construction, drilling, completion
and/or fracture stimulation of oil and natural gas wells thus
becomes obvious. Consequently, while such uses constitute only a
small fraction of the applications of stiff and strong materials,
they provide fertile territory for the development of new or
improved materials and manufacturing processes for the fabrication
of such materials.
[0007] We will focus much of the remaining discussion of the
background of the invention on the use of particulate materials as
proppants. One key measure of end use performance of proppants is
the retention of high conductivity of liquids and gases through
packings of the particles in aggressive environments under high
compressive loads at elevated temperatures.
[0008] The use of stiff and strong solid proppants has a long
history in the oil and natural gas industry. Throughout most of
this history, particles made from polymeric materials (including
crosslinked polymers) have been considered to be unsuitable for use
by themselves as proppants. The reason for this prejudice is the
perception that polymers are too deformable, as well as lacking in
the ability to withstand the combination of elevated compressive
loads, temperatures and aggressive environments that are commonly
encountered in oil and natural gas wells.
[0009] Consequently, work on proppant material development has
focused mainly on sands, on ceramics, and on sands and ceramics
coated by crosslinked polymers to improve some aspects of their
performance. This situation has prevailed despite the fact that
most polymers have densities that are much closer to that of water
so that in particulate form they can be transported much more
readily into a fracture by low-density fracturing or carrier fluids
such as unviscosified water.
[0010] Nonetheless, the obvious practical advantages [see a review
by Edgeman (2004)] of developing the ability to use lightweight
particles that possess almost neutral buoyancy relative to water
have stimulated a considerable amount of work over the years.
However, as will be seen from the review of the prior art provided
below, progress in this field of invention has been very slow as a
result of the many technical challenges that exist to the
successful development of cost-effective lightweight particles that
possess sufficient stiffness, strength and heat resistance.
B. Prior Art
[0011] The prior art can be described most clearly, and hence the
invention can be placed in the proper context most effectively, by
subdividing this section into four subsections. The first
subsection will describe prior art related to the development of
"as-polymerized" thermoset polymer particles. The second subsection
will describe prior art related to the development of thermoset
polymer particles that are subjected to post-polymerization heat
treatment. The third subsection will describe prior art related to
the development of thermoset polymer composite particles where the
particles are reinforced by conventional fillers. The fourth
subsection will describe prior art related to the development of
ceramic nanocomposite particles where a ceramic matrix is
reinforced by nanofillers.
1. "as-Polymerized" Thermoset Polymer Particles
[0012] As discussed above, particles made from polymeric materials
have historically been considered to be unsuitable for use by
themselves as proppants. Consequently, their past uses in proppant
materials have focused mainly on their placement as coatings on
sands and ceramics, in order to improve some aspects of the
performance of the sand and ceramic proppants.
[0013] Significant progress was made in the use of crosslinked
polymeric particles themselves as constituents of proppant
formulations in prior art taught by Rickards, et al. (U.S. Pat. No.
6,059,034; U.S. Pat. No. 6,330,916). However, these inventors still
did not consider or describe the polymeric particles as proppants.
Their invention only related to the use of the polymer particles in
blends with particles of more conventional proppants such as sands
or ceramics. They taught that the sand or ceramic particles are the
proppant particles, and that the "deformable particulate material"
consisting of polymer particles mainly serves to improve the
fracture conductivity, reduce the generation of fines and/or reduce
proppant flowback relative to the unblended sand or ceramic
proppants. Thus while their invention differs significantly from
the prior art in the sense that the polymer is used in particulate
form rather than being used as a coating, it shares with the prior
art the limitation that the polymer still serves merely as a
modifier improving the performance of a sand or ceramic proppant
rather than being considered for use as a proppant in its own
right.
[0014] Bienvenu (U.S. Pat. No. 5,531,274) disclosed progress
towards the development of lightweight proppants consisting of
high-strength crosslinked polymeric particles for use in hydraulic
fracturing applications. However, embodiments of this prior art,
based on the use of styrene-divinylbenzene (S-DVB) copolymer beads
manufactured by using conventional fabrication technology and
purchased from a commercial supplier, failed to provide an
acceptable balance of performance and price. They cost far more
than the test standard (Jordan sand) while being outperformed by
Jordan sand in terms of the liquid conductivity and liquid
permeability characteristics of their packings measured according
to the industry-standard API RP 61 testing procedure. [This
procedure is described by the American Petroleum Institute in its
publication titled "Recommended Practices for Evaluating Short Term
Proppant Pack Conductivity" (first edition, Oct. 1, 1989).] The
need to use a very large amount of an expensive crosslinker (50 to
80% by weight of DVB) in order to obtain reasonable performance
(not too inferior to that of Jordan Sand) was a key factor in the
higher cost that accompanied the lower performance.
[0015] The most advanced prior art in stiff and strong crosslinked
polymer particle technologies for use in applications in oil and
natural gas drilling was developed by Albright (U.S. Pat. No.
6,248,838) who taught the concept of a "rigid chain entanglement
crosslinked polymer". In summary, the reactive formulation and the
processing conditions were modified to achieve "rapid rate
polymerization". While not improving the extent of covalent
crosslinking relative to conventional isothermal polymerization,
rapid rate polymerization results in the "trapping" of an unusually
large number of physical entanglements in the polymer. These
additional entanglements can result in a major improvement of many
properties. For example, the liquid conductivities of packings of
S-DVB copolymer beads with w.sub.DVB=0.2 synthesized via rapid rate
polymerization are comparable to those that were found by Bienvenu
(U.S. Pat. No. 5,531,274) for packings of conventionally produced
S-DVB beads at the much higher DVB level of w.sub.DVB=0.5. Albright
(U.S. Pat. No. 6,248,838) thus provided the key technical
breakthrough that enabled the development of the first generation
of crosslinked polymer beads possessing sufficiently attractive
combinations of performance and price characteristics to result in
their commercial use in their own right as solid polymeric
proppants.
2. Heat-Treated Thermoset Polymer Particles
[0016] There is no prior art that relates to the development of
heat-treated thermoset polymer particles for use in oil and natural
gas well construction applications. One needs to look into another
field of technology to find prior art of some relevance. Nishimori,
et. al. (JP1992-22230) focused on the development of particles for
use in liquid crystal display panels. They taught the use of
post-polymerization heat treatment to increase the compressive
elastic modulus of S-DVB particles at room temperature. They only
claimed compositions polymerized from reactive monomer mixtures
containing 20% or more by weight of DVB or other crosslinkable
monomer(s) prior to the heat treatment. They stated explicitly that
improvements obtained with lower weight fractions of the
crosslinkable monomer(s) were insufficient and that hence such
compositions were excluded from the scope of their patent.
3. Thermoset Polymer Composite Particles
[0017] This subsection will be easier to understand if it is
further subdivided into two subsections. As was discussed above,
the prior art on the use of polymers as components of proppant
particles has focused mainly on the development of thermoset
polymer coatings for rigid inorganic materials such as sand or
ceramic particles. These types of heterogeneous (composite)
particles will be discussed in the first subsection. Composite
particles where the thermoset polymer plays a role that goes beyond
that of a coating will be discussed in the second subsection.
[0018] a. Thermoset Polymers as Coatings
[0019] The prior art discussed in this subsection is mainly of
interest for historical reasons, as examples of the evolution of
the use of thermoset polymers as components in composite proppant
particles.
[0020] Underdown, et al. (U.S. Pat. No. 4,443,347) and of Glaze, et
al. (U.S. Pat. No. 4,664,819) taught the coating of particles such
as silica sand or glass beads with a thermoset polymer (such as a
phenol-formaldehyde resin) that is cured fully (in their
terminology, "pre-cured") prior to the injection of a proppant
charge consisting of such particles into a well.
[0021] An interesting alternative coating technology was taught by
Graham, et al. (U.S. Pat. No. 4,585,064) who developed resin-coated
particles comprising a particulate substrate, a substantially cured
inner resin coating, and a heat-curable outer resin coating.
According to their teaching, the outer resin coating should cure,
and should thus enable the particles to form a coherent mass
possessing the desired level of liquid conductivity, under the
temperatures and compressive loads found in subterranean
formations. However, it is not difficult to anticipate the many
technical difficulties that can arise in attempting to reduce such
an approach reliably and consistently to practice.
[0022] b. Thermoset Polymers as Matrix Phase Containing Dispersed
Finely Divided Filler Material
[0023] McDaniel, et al. (U.S. Pat. No. 6,632,527) describes
composite particles made of a binder and filler; for use in
subterranean formations (for example, as proppants and as gravel
pack components), in water filtration, and in artificial turf for
sports fields. The filler consists of finely divided mineral
particles that can be of any available composition. Fibers are also
used in some embodiments as optional fillers. The sizes of the
filler particles are required to fall within the range of 0.5
microns to 60 microns. The proportion of filler in the composite
particle is very large (60% to 90% by volume). The binder
formulation is required to include at least one member of the group
consisting of inorganic binder, epoxy resin, novolac resin, resole
resin, polyurethane resin, alkaline phenolic resole curable with
ester, melamine resin, urea-aldehyde resin, urea-phenol-aldehyde
resin, furans, synthetic rubber, and/or polyester resin. The final
thermoset polymer composite particles of the required size and
shape are obtained by a succession of process steps such as the
mixing of a binder stream with a filler particle stream,
agglomerative granulation, and the curing of granulated material
streams.
4. Ceramic Nanocomposite Particles
[0024] Nguyen, et al. (U.S. 20050016726) taught the development of
ceramic nanocomposite particles comprising a base material (present
at roughly 50% to 90% by weight) and at least one nanoparticle
material (present at roughly 0.1% to 30% by weight). Optionally, a
polymeric binder, an organosilane coupling agent, and/or hollow
microspheres, can also be included. The base material comprises
clay, bauxite, alumina, silica, or mixtures thereof. It is stated
that a suitable method for forming the composite particulates from
the dry ingredients is to sinter by heating at a temperature of
between roughly 1000.degree. C. and 2000.degree. C., which is a
ceramic fabrication process. Given the types of formulation
ingredients used as base materials by Nguyen, et al. (U.S.
20050016726), and furthermore the fact that even if they were to
incorporate a polymeric binder in an embodiment of their invention
said polymeric binder would not retain its normal chemical
composition and polymer chain structure when a particulate is
sintered by heating it at a temperature of between 1000.degree. C.
and about 2000.degree. C., their composite particulates consist of
the nanofiller(s) dispersed in a ceramic matrix.
C. Scientific Literature
[0025] The development of thermoset polymer nanocomposites requires
the consideration of a vast and multidisciplinary range of polymer
and composite materials science and chemistry challenges. It is
essential to convey these challenges in the context of the
fundamental scientific literature.
[0026] Bicerano (2002) provides a broad overview of polymer and
composite materials science that can be used as a general reference
for most aspects of the following discussion. Many additional
references will also be provided below, to other publications which
treat specific issues in greater detail than what could be
accommodated in Bicerano (2002).
[0027] 1. Selected Fundamental Aspects of the Curing of Crosslinked
Polymers
[0028] It is essential, first, to review some fundamental aspects
of the curing of crosslinked polymers, which are applicable to such
polymers regardless of their form (particulate, coating, or
bulk).
[0029] The properties of crosslinked polymers prepared by standard
manufacturing processes are often limited by the fact that such
processes typically result in incomplete curing. For example, in an
isothermal polymerization process, as the glass transition
temperature (T.sub.g) of the growing polymer network increases, it
may reach the polymerization temperature while the reaction is
still in progress. If this happens, then the molecular motions slow
down significantly so that further curing also slows down
significantly. Incomplete curing yields a polymer network that is
less densely crosslinked than the theoretical limit expected from
the functionalities and relative amounts of the starting reactants.
For example, a mixture of monomers might contain 80% DVB by weight
as a crosslinker but the final extent of crosslinking that is
attained may not be much greater than what was attained with a much
smaller percentage of DVB. This situation results in lower
stiffness, lower strength, lower heat resistance, and lower
environmental resistance than the thermoset is capable of
manifesting when it is fully cured and thus maximally
crosslinked.
[0030] When the results of the first scan and the second scan of
S-DVB beads containing various weight fractions of DVB (w.sub.DVB),
obtained by Differential Scanning calorimetry (DSC), as reported by
Bicerano, et al. (1996) (see FIG. 1) are compared, it becomes clear
that the low performance and high cost of the "as purchased" S-DVB
beads utilized by Bienvenu (U.S. Pat. No. 5,531,274) are related to
incomplete curing. This incomplete curing results in the
ineffective utilization of DVB as a crosslinker and thus in the
incomplete development of the crosslinked network. In summary,
Bicerano, et al. (1996), showed that the T.sub.g of typical
"as-polymerized" 5-DVB copolymers, as measured by the first DSC
scan, increased only slowly with increasing w.sub.DVB, and
furthermore that the rate of further increase of T.sub.g slowed
down drastically for w.sub.DVB>0.08. By contrast, in the second
DSC scan (performed on S-DVB specimens whose curing had been driven
much closer to completion as a result of the temperature ramp that
had been applied during the first scan), T.sub.g grew much more
rapidly with w.sub.DVB over the entire range of up to
w.sub.DVB=0.2458 that was studied. The more extensively cured
samples resulting from the thermal history imposed by the first DSC
scan can, thus, be considered to provide much closer approximations
to the ideal theoretical limit of a "fully cured" polymer
network.
[0031] 2. Effects of Heat Treatment on Key Properties of Thermoset
Polymers
[0032] a. Maximum Possible Use Temperature
[0033] As was illustrated by Bicerano, et al. (1996) for S-DVB
copolymers with w.sub.DVB of up to 0.2458, enhancing the state of
cure of a thermoset polymer network can increase T.sub.g very
significantly relative to the T.sub.g of the "as-polymerized"
material. In practice, the heat distortion temperature (HDT) is
used most often as a practical indicator of the softening
temperature of a polymer under load. As was shown by Takemori
(1979), a systematic understanding of the HDT is possible through
its direct correlation with the temperature dependences of the
tensile (or equivalently, compressive) and shear elastic moduli.
For amorphous polymers, the precipitous decrease of these elastic
moduli as T.sub.g is approached from below renders the HDT
well-defined, reproducible, and predictable. HDT is thus closely
related to (and usually slightly lower than) T.sub.g for amorphous
polymers, so that it can be increased significantly by increasing
T.sub.g significantly.
[0034] The HDT decreases gradually with increasing magnitude of the
load used in its measurement. For example, for general-purpose
polystyrene (which has T.sub.g=100.degree. C.), HDT=95.degree. C.
under a load of 0.46 MPa and HDT=85.degree. C. under a load of 1.82
MPa are typical values. However, the compressive loads deep in an
oil well or natural gas well are normally far higher than the
standard loads (0.46 MPa and 1.82 MPa) used in measuring the HDT.
Consequently, amorphous thermoset polymer particles can be expected
to begin to deform significantly at a lower temperature than the
HDT of the polymer measured under the standard high load of 1.82
MPa. This deformation will cause a decrease in the conductivities
of liquids and gases through the propped fracture, and hence in the
loss of effectiveness as a proppant, at a somewhat lower
temperature than the HDT value of the polymer measured under the
standard load of 1.82 MPa.
[0035] b. Mechanical Properties
[0036] As was discussed earlier, Nishimori, et. al. (JP1992-22230)
used heat treatment to increase the compressive elastic modulus of
their S-DVB particles (intended for use in liquid crystal display
panels) significantly at room temperature (and hence far below
T.sub.g). Deformability under a compressive load is inversely
proportional to the compressive elastic modulus. It is, therefore,
important to consider whether one may also anticipate major
benefits from heat treatment in terms of the reduction of the
deformability of thermoset polymer particles intended for oil and
natural gas drilling applications, when these particles are used in
subterranean environments where the temperature is far below the
T.sub.g of the particles. As explained below, the enhancement of
curing via post-polymerization heat treatment is generally expected
to have a smaller effect on the compressive elastic modulus (and
hence on the proppant performance) of thermoset polymer particles
when used in oil and natural gas drilling applications at
temperatures far below their T.sub.g.
[0037] Nishimori, et. al. (JP1992-22230) used very large amounts of
DVB (w.sub.DVB>>0.2). By contrast, much smaller amounts of
DVB (w.sub.DVB.ltoreq.0.2) must be used for economic reasons in the
"lower value" oil and natural gas drilling applications. The
elastic moduli of a polymer at temperatures far below T.sub.g are
determined primarily by deformations that are of a rather local
nature and hence on a short length scale. Some enhancement of the
crosslink density via further curing (when the network junctions
created by the crosslinks are far away from each other to begin
with) will hence not normally have nearly as large an effect on the
elastic moduli as when the network junctions are very close to each
other to begin with and then are brought even closer by the
enhancement of curing via heat treatment. Consequently, while the
compressive elastic modulus can be expected to increase
significantly upon heat treatment when w.sub.DVB is very large, any
such effect will normally be less pronounced at low values of
w.sub.DVB. In summary, it can thus generally be expected that the
enhancement of the compressive elastic modulus at temperatures far
below T.sub.g will probably be small for the types of formulations
that are most likely to be used in the synthesis of thermoset
polymer particles for oil and natural gas drilling
applications.
[0038] 3. Effects of Nanoparticle Incorporation on Key Properties
of Thermoset Polymers
[0039] a. Maximum Possible Use Temperature
[0040] As was pointed out by Takemori (1979), the addition of rigid
fillers has a negligible effect on the HDT of amorphous polymers.
However, nanocomposite materials and technologies had not yet been
developed in 1979. It is, hence, important to consider, based on
the data that have been gathered and the insights that have been
obtained more recently, whether nanofillers may be expected to
behave in a qualitatively different manner because of their
geometric characteristics.
[0041] A review article by Aharoni (1998) considered this question
and showed that three criteria must be considered. Here are the
most relevant excerpts from his article: "When a combination of the
following three conditions is fulfilled, then the glass transition
temperature . . . may be increased relative to that of the same
polymer in the absence of these three conditions . . . . First,
very large surface area of a rigid heterogeneous material in close
contact with the amorphous phase of the polymer. Such large surface
areas may be obtained by having a rigid additive material extremely
finely ground, preferably to nanometer length scale. Second, strong
attractive interactions should exist between the heterogeneous
surfaces and the polymer. In the absence of strong attractive
interactions with the heterogeneous rigid surfaces, the chain
segments in the boundary layer are capable of relaxing to a state
approximating the bulk polymer and the T.sub.g will be identical or
very slightly higher than that of the pure bulk polymer. Third,
measure of motional cooperation must exist between interchain and
intrachain fragments. Unlike the effects of high modulus
heterogeneous additives on the averaged modulus of the system in
which they are present, the elevation of T.sub.g of the polymer
matrix was repeatedly shown to require not only that the polymer
itself will be a high molecular weight substance, but that the
additive will be finely comminuted to generate very large
polymer-heterophase interfacial surface area, and, especially
important, that strong attractive interactions will exist between
the polymer and the foreign additive. These interactions are
generally of an ionic, hydrogen bonding, or dipolar nature and, as
a rule, require that the foreign additive will have surface energy
higher than or at least equal to, but never lower than, that of the
amorphous polymer in which it is being incorporated."
[0042] Almost by definition, Aharoni's first condition will be
satisfied for any nanofiller that has been dispersed well in the
polymer matrix. Furthermore, since a thermoset polymer contains a
covalently bonded three-dimensional network structure, his third
condition will also be satisfied if any thermoset polymer is used
as the matrix material. However, in most systems, there will not be
strong attractive interactions "generally of an ionic, hydrogen
bonding, or dipolar nature" between the polymer and the nanofiller,
so that the second criterion will not be satisfied. It can,
therefore, be concluded that, for most combinations of polymer and
nanofiller, T.sub.g will not increase significantly upon
incorporation of the nanofiller so that the maximum possible use
temperature will not increase significantly either. There will,
however, be exceptions to this general rule. Combinations of
polymer and nanofiller that manifest strong attractive interactions
can be found, and for such combinations both T.sub.g and the
maximum possible use temperature can increase significantly upon
nanofiller incorporation.
[0043] b. Mechanical Properties
[0044] It is well-established that the incorporation of rigid
fillers into a polymer matrix can produce a composite material
which has significantly greater stiffness (elastic modulus) and
strength (stress required to induce failure) than the base polymer.
It is also well-established that rigid nanofillers can generally
stiffen and strengthen a polymer matrix more effectively than
conventional rigid fillers of similar composition since their
geometries allow them to span (or "percolate through") a polymer
specimen at much lower volume fractions than conventional fillers.
This particular advantage of nanofillers over conventional fillers
is well-established and a major driving force for the vast research
and development effort worldwide to develop new nanocomposite
products.
[0045] FIG. 2 provides an idealized schematic illustration of the
effectiveness of nanofillers in terms of their ability to
"percolate through" a polymer specimen even when they are present
at a low volume fraction. It is important to emphasize that FIG. 2
is of a completely generic nature. It is presented merely to
facilitate the understanding of nanofiller percolation, without
implying that it provides an accurate depiction of the expected
behavior of any particular nanofiller in any particular polymer
matrix. In practice, the techniques of electron microscopy are
generally used to observe the morphologies of actual embodiments of
the nanocomposite concept. Specific examples of the ability of
nanofillers such as carbon black and fumed silica to "percolate" at
extremely low volume fractions when dispersed in polymers are
provided by Zhang, et al (2001). The vast literature and trends on
the dependences of percolation thresholds and packing fractions on
particle shape, aggregation, and other factors, are reviewed by
Bicerano, et al. (1999).
[0046] As has also been studied extensively [for example, see
Okamoto, et al. (1999)] but is less widely recognized by workers in
the field, the incorporation of rigid fillers of appropriate types
and dimensions in the right amount (often just a very small volume
fraction) can toughen a polymer in addition to stiffening it and
strengthening it. "Toughening" implies a reduction in the tendency
to undergo brittle fracture. If and when it is realized for
proppant particles, it is an important additional benefit since it
reduces the risk of the generation of "fines" during use.
[0047] 4. Technical Challenges to Nanoparticle Incorporation in
Thermoset Polymers
[0048] It is important to also review the many serious technical
challenges that exist to the successful incorporation of
nanoparticles in thermoset polymers. Appreciation of these
obstacles can help workers in the field of the invention gain a
better understanding of the invention. There are three major types
of potential obstacles. In general, each potential obstacle will
tend to become more serious with increasing nanofiller volume
fraction, so that it is usually easier to incorporate a small
volume fraction of a nanofiller into a polymer than it is to
incorporate a larger volume fraction. This subsection is subdivided
further into the following three subsections where each type of
major potential obstacle will be discussed in turn.
[0049] a. Difficulty of Dispersing Nanofiller
[0050] The most common difficulty that is encountered in preparing
polymer nanocomposites involves the need to disperse the
nanofiller. The specific details of the source and severity of the
difficulty, and of the methods that may help overcome the
difficulty, differ between types of nanofillers, polymers, and
fabrication processes (for example, the "in situ" synthesis of the
polymer in an aqueous or organic medium containing the nanofiller,
versus the addition of the nanofiller into a molten polymer).
However, some important common aspects can be identified.
[0051] Most importantly, nanofiller particles of the same kind
often have strong attractive interactions with each other. As a
result, they tend to "clump together"; for example, preferably into
agglomerates (if the nanofiller is particulate), bundles (if the
nanofiller is fibrous), or stacks (if the nanofiller is discoidal).
In most systems, their attractive interactions with each other are
stronger than their interactions with the molecules constituting
the dispersing medium, so that their dispersion is
thermodynamically disfavored and hence extremely difficult.
[0052] Even in systems where the dispersion of the nanofillers is
thermodynamically favored, it is often still very difficult to
achieve because of the large kinetic barriers (activation energies)
that must be surmounted. Consequently, nanofillers are very rarely
easy to disperse in a polymer.
[0053] b. High Dispersion Viscosity
[0054] Another difficulty with the fabrication of nanocomposites is
the fact that, once the nanofiller is dispersed in the appropriate
medium (for example, an aqueous or organic medium containing the
nanofiller for the "in situ" synthesis of the polymer, or a molten
polymer into which nanofiller is added), the viscosity of the
resulting dispersion may (and often does) become very high. When
this happens, it can impede the successful execution of the
fabrication process steps that must follow the dispersion of the
nanofiller to complete the preparation of the nanocomposite.
[0055] Dispersion rheology is a vast area of both fundamental and
applied research. It dates back to the 19.sup.th century, so that
there is a vast collection of data and a good fundamental
understanding of the factors controlling the viscosities of
dispersions. Nonetheless, it is still at the frontiers of materials
science, so that major new experimental and theoretical progress is
continuing to be made. In fact, the advent of nanotechnology, and
the frequent emergence of high dispersion viscosity as an obstacle
to the fabrication of polymer nanocomposites, have been
instrumental in advancing the state of the art in this field.
Bicerano, et al. (1999) have provided a comprehensive overview
which can serve as a resource for workers interested in learning
more about this topic.
[0056] c. Interference with Polymerization and Network
Formation
[0057] An additional potential difficulty may be encountered in
systems where chemical Reactions are taking place in a medium
containing a nanofiller. This is the possibility that the
nanofiller may have an adverse effect on the chemical reactions. As
can reasonably be expected, any such adverse effects can be far
more severe in systems where polymerization and network formation
take place simultaneously in the presence of a nanofiller than they
can in systems where preformed polymer chains are crosslinked in
the presence of a nanofiller. The preparation of an S-DVB
nanocomposite via suspension polymerization in a medium containing
a nanofiller is an example of a process where polymerization and
network formation both take place in the presence of a nanofiller.
On the other hand, the vulcanization of a nanofilled rubber is a
process where preformed polymer chains are crosslinked in the
presence of a nanofiller.
[0058] The combined consideration of the work of Lipatov, et al.
(1966,1968), Popov, et al. (1982), and Bryk, et al. (1985, 1986,
1988) helps in providing a broad perspective into the nature of the
difficulties that may arise. To summarize, the presence of a filler
with a high specific surface area can disrupt both polymerization
and network formation in a process such as the suspension
polymerization of an S-DVB copolymer nanocomposite. These outcomes
can arise from the combined effects of the adsorption of initiators
on the surfaces of the nanofiller particles and the interactions of
the growing polymer chains with the nanofiller surfaces. Adsorption
on the nanofiller surface can affect the rate of thermal
decomposition of the initiator. Interactions of the growing polymer
chains with the nanofiller surfaces can result both in the
reduction of the mobility of growing polymer chains and in their
breakage. Very strong attractions between the initiator and the
nanofiller surfaces (for example, the grafting of the initiators on
the nanofiller surfaces) can potentially augment all of these
detrimental effects.
[0059] Taguchi, et al. (1999) provided a fascinating example of how
drastically the formulation can affect the particle morphology.
They described the results obtained by adding hydrophilic fine
powders [nickel (Ni) of mean particle size 0.3 microns, indium
oxide (In.sub.2O.sub.3) of mean particle size 0.03 microns, and
magnetite (Fe.sub.3O.sub.4) of mean particle size 0.1, 0.3 or 0.9
microns] to the aqueous phase during the suspension polymerization
of S-DVB. These particles had such a strong affinity to the aqueous
phase that they did not even go inside the S-DVB beads. Instead,
they remained entirely outside the beads. Consequently, the
composite particles consisted of S-DVB beads whose surfaces were
uniformly covered by a coating of inorganic powder. Furthermore,
these S-DVB beads rapidly became smaller with increasing amount of
powder at a fixed powder particle diameter, as well as with
decreasing powder particle diameter (and hence increasing number
concentration of powder particles) at a given powder weight
fraction.
SUMMARY OF THE INVENTION
[0060] The present invention involves a novel approach towards the
practical development of stiff, strong, tough, heat resistant, and
environmentally resistant ultralightweight particles, for use in
the construction, drilling, completion and/or fracture stimulation
of oil and natural gas wells.
[0061] The disclosure is summarized below in three key aspects: (A)
Compositions of Matter (thermoset nanocomposite particles that
exhibit improved properties compared with prior art), (B) Processes
(methods for manufacture of said compositions of matter), and (C)
Applications (utilization of said compositions of matter in the
construction, drilling, completion and/or fracture stimulation of
oil and natural gas wells).
[0062] The disclosure describes lightweight thermoset nanocomposite
particles whose properties are improved relative to prior art. The
particles targeted for development include, but are not limited to,
terpolymers of styrene, ethyvinylbenzene and divinylbenzene;
reinforced by particulate carbon black of nanoscale dimensions. The
particles exhibit any one or any combination of the following
properties: enhanced stiffness, strength, heat resistance, and/or
resistance to aggressive environments; and/or improved retention of
high conductivity of liquids and/or gases through packings of said
particles when said packings are placed in potentially aggressive
environments under high compressive loads at elevated
temperatures.
[0063] The disclosure also describes processes that can be used to
manufacture said particles. The fabrication processes targeted for
development include, but are not limited to, suspension
polymerization in the presence of nanofiller, and optionally
post-polymerization heat treatment with said particles still in the
reactor fluid that remains after the suspension polymerization to
further advance the curing of the matrix polymer.
[0064] The disclosure finally describes the use of said particles
in practical applications. The targeted applications include, but
are not limited to, the construction, drilling, completion and/or
fracture stimulation of oil and natural gas wells; for example, as
a proppant partial monolayer, a proppant pack, an integral
component of a gravel pack completion, a ball bearing, a solid
lubricant, a drilling mud constituent, and/or a cement
additive.
A. Compositions of Matter
[0065] The compositions of matter of the present invention are
thermoset polymer nanocomposite particles where one or optionally
more than one type of nanofiller is intimately embedded in a
polymer matrix. Any additional formulation component(s) familiar to
those skilled in the art can also be used during the preparation of
said particles; such as initiators, catalysts, inhibitors,
dispersants, stabilizers, rheology modifiers, buffers,
antioxidants, defoamers, impact modifiers, plasticizers, pigments,
flame retardants, smoke retardants, or mixtures thereof. Some of
the said additional component(s) may also become either partially
or completely incorporated into said particles in some embodiments
of the invention. However, the two required major components of
said particles are a thermoset polymer matrix and at least one
nanofiller. Hence this subsection will be further subdivided into
three subsections. Its first subsection will teach the volume
fraction of nanofiller(s) that may be used in the particles of the
invention. Its second subsection will teach the types of thermoset
polymers that may be used as matrix materials. Its third subsection
will teach the types of nanofillers that may be incorporated.
1. Nanofiller Volume Fraction
[0066] By definition, a nanofiller possesses at least one principal
axis dimension whose length is less than 0.5 microns (500
nanometers). This geometric attribute is what differentiates a
nanofiller from a finely divided conventional filler, such as the
fillers taught by McDaniel, et al. (U.S. Pat. No. 6,632,527) whose
characteristic lengths ranged from 0.5 microns to 60 microns.
[0067] The dispersion of a nanofiller in a polymer is generally
more difficult than the dispersion of a conventional filler of
similar chemical composition in the same polymer. However, if
dispersed properly during composite particle fabrication,
nanofillers can reinforce the matrix polymer far more efficiently
than conventional fillers. Consequently, while 60% to 90% by volume
of filler is claimed by McDaniel, et al. (U.S. Pat. No. 6,632,527),
only 0.001% to 60% by volume of nanofiller is claimed in the
present invention.
[0068] Without reducing the generality of the present invention, a
nanofiller volume fraction of 0.1% to 15% is used in its currently
preferred embodiments.
2. Matrix Polymers
[0069] Any rigid thermoset polymer may be used as the matrix
polymer of the present invention. Rigid thermoset polymers are, in
general, amorphous polymers where covalent crosslinks provide a
three-dimensional network. However, unlike thermoset elastomers
(often referred to as "rubbers") which also possess a
three-dimensional network of covalent crosslinks, the rigid
thermosets are, by definition, "stiff". In other words, they have
high elastic moduli at "room temperature" (25.degree. C.), and
often up to much higher temperatures, because their combinations of
chain segment stiffness and crosslink density result in a high
glass transition temperature.
[0070] Some examples of rigid thermoset polymers that can be used
as matrix materials of the invention will be provided below. It is
to be understood that these examples are being provided without
reducing the generality of the invention, merely to facilitate the
teaching of the invention.
[0071] Rigid thermoset polymers that are often used as matrix
(often referred to as "binder") materials in composites include,
but are not limited to, crosslinked epoxies, epoxy vinyl esters,
polyesters, phenolics, polyurethanes, and polyureas. Rigid
thermoset polymers that are used less often because of their high
cost despite their exceptional performance include, but are not
limited to, crosslinked polyimides. These various types of polymers
can, in different embodiments of the invention, be prepared by
starting either from their monomers, or from oligomers that are
often referred to as "prepolymers", or from suitable mixtures of
monomers and oligomers.
[0072] Many additional types of rigid thermoset polymers can also
be used as matrix materials in composites, and are all within the
scope of the invention. Such polymers include, but are not limited
to, various families of crosslinked copolymers prepared most often
by the polymerization of vinylic monomers, of vinylidene monomers,
or of mixtures thereof.
[0073] The "vinyl fragment" is commonly defined as the
CH.sub.2.dbd.CH-- fragment. So a "vinylic monomer" is a monomer of
the general structure CH.sub.2.dbd.CHR where R can be any one of a
vast variety of molecular fragments or atoms (other than hydrogen).
When a vinylic monomer CH.sub.2.dbd.CHR reacts, it is incorporated
into the polymer as the --CH.sub.2--CHR-- repeat unit. Among rigid
thermosets built from vinylic monomers, the crosslinked styrenics
and crosslinked acrylics are especially familiar to workers in the
field. Some other familiar types of vinylic monomers (among others)
include the olefins, vinyl alcohols, vinyl esters, and vinyl
halides.
[0074] The "vinylidene fragment" is commonly defined as the
CH.sub.2.dbd.CR''-- fragment. So a "vinylidene monomer" is a
monomer of the general structure CH.sub.2.dbd.CR'R'' where R' and
R'' can each be any one of a vast variety of molecular fragments or
atoms (other than hydrogen). When a vinylidene monomer
CH.sub.2.dbd.CR'R'' reacts, it is incorporated into a polymer as
the --CH.sub.2--CR'R''-repeat unit. Among rigid thermosets built
from vinylidene polymers, the crosslinked alkyl acrylics [such as
crosslinked poly(methyl methacrylate)] are especially familiar to
workers in the field. However, vinylidene monomers similar to each
type of vinyl monomer (such as the styrenics, acrylates, olefins,
vinyl alcohols, vinyl esters and vinyl halides, among others) can
be prepared. One example of particular interest in the context of
styrenic monomers is .alpha.-methyl styrene, a vinylidene-type
monomer that differs from styrene (a vinyl-type monomer) by having
a methyl (--CH.sub.3) group serving as the R'' fragment replacing
the hydrogen atom attached to the .alpha.-carbon.
[0075] Thermosets based on vinylic monomers, on vinylidene
monomers, or on mixtures thereof, are typically prepared by the
reaction of a mixture containing one or more non-crosslinking
(difunctional) monomer and one or more crosslinking (three or
higher functional) monomers. All variations in the choices of the
non-crosslinking monomer(s), the crosslinking monomers(s), and
their relative amounts [subject solely to the limitation that the
quantity of the crosslinking monomer(s) must not be less than 1% by
weight], are within the scope of the invention.
[0076] Without reducing the generality of the invention, in its
currently preferred embodiments, the thermoset matrix consists of a
terpolymer of styrene (non-crosslinking), ethyvinylbenzene (also
non-crosslinking), and divinylbenzene (crosslinking), with the
weight fraction of divinylbenzene ranging from 3% to 35% by weight
of the starting monomer mixture.
3. Nanofillers
[0077] By definition, a nanofiller possesses at least one principal
axis dimension whose length is less than 0.5 microns (500
nanometers). Some nanofillers possess only one principal axis
dimension whose length is less than 0.5 microns. Other nanofillers
possess two principal axis dimensions whose lengths are less than
0.5 microns. Yet other nanofillers possess all three principal axis
dimensions whose lengths are less than 0.5 microns. Any reinforcing
material possessing one nanoscale dimension, two nanoscale
dimensions, or three nanoscale dimensions, can be used as the
nanofiller in embodiments of the invention. Any mixture of two or
more different types of such reinforcing materials can also be used
as the nanofiller in embodiments of the invention.
[0078] Some examples of nanofillers that can be incorporated into
the nanocomposites of the invention will be provided below. It is
to be understood that these examples are being provided without
reducing the generality of the invention, merely to facilitate the
teaching of the invention.
[0079] Nanoscale carbon black, fumed silica and fumed alumina, such
as products of these types that are currently being manufactured by
the Cabot Corporation, consist of aggregates of small primary
particles. See FIG. 3 for a schematic illustration of such an
aggregate, and of a larger agglomerate. The aggregates may contain
many very small primary particles, often arranged in a "fractal"
pattern, resulting in aggregate principal axis dimensions that are
also shorter than 0.5 microns. These aggregates (and not the
individual primary particles that constitute them) are, in general,
the smallest units of these nanofillers that are dispersed in a
polymer matrix under normal fabrication conditions. The available
grades of such nanofillers include variations in specific surface
area, extent of branching (structure) in the aggregates, and
chemical modifications intended to facilitate dispersion in
different types of media (such as aqueous or organic mixtures).
Some product types of such nanofillers are also provided in
"fluffy" grades of lower bulk density that are easier to disperse
than the base grade but less convenient to transport and store
since the same weight of material occupies more volume when it is
in its fluffy form. Some products grades of such nanofillers are
also provided pre-dispersed in an aqueous medium.
[0080] Carbon nanotubes, carbon nanofibers, and cellulosic
nanofibers constitute three other classes of nanofillers. When
separated from each other by breaking up the bundles in which they
are often found and then dispersed well in a polymer, they serve as
fibrous reinforcing agents. In different products grades, they may
have two principal axis dimensions in the nanoscale range (below
500 nanometers), or they may have all three principal axis
dimensions in the nanoscale range (if they have been prepared by a
process that leads to the formation of shorter nanotubes or
nanofibers). Currently, carbon nanotubes constitute the most
expensive nanofillers of fibrous shape. Carbon nanotubes are
available in single-wall and multi-wall versions. The single-wall
versions offer the highest performance, but currently do so at a
much higher cost than the multi-wall versions. Nanotubes prepared
from inorganic materials (such as boron nitride) are also
available.
[0081] Natural and synthetic nanoclays constitute another major
class of nanofiller. Nanocor and Southern Clay Products are the two
leading suppliers of nanoclays at this time. When "exfoliated"
(separated from each other by breaking up the stacks in which they
are normally found) and dispersed well in a polymer, the nanoclays
serve as discoidal (platelet-shaped) reinforcing agents. The
thickness of an individual platelet is around one nanometer (0.001
microns). The lengths in the other two principal axis dimensions
are much larger. They range between 100 and 500 nanometers in many
product grades, thus resulting in a platelet-shaped nanofiller that
has three nanoscale dimensions. They exceed 500 nanometers, and
thus result in a nanofiller that has only one nanoscale dimension,
in some other grades.
[0082] Many additional types of nanofillers are also available;
including, but not limited to, very finely divided grades of fly
ash, the polyhedral oligomeric silsesquioxanes, and clusters of
different types of metals, metal alloys, and metal oxides. Since
the development of nanofillers is an area that is at the frontiers
of materials research and development, the future emergence of yet
additional types of nanofillers that are not currently known may
also be readily anticipated.
[0083] Without reducing the generality of the invention, in its
currently preferred embodiments, nanoscale carbon black grades
supplied by Cabot Corporation are being used as the nanofiller.
B. Processes
[0084] In most cases, the incorporation of a nanofiller into the
thermoset polymer matrix will increase the compressive elastic
modulus uniformly throughout the entire use temperature range
(albeit usually not by exactly the same factor at each
temperature), while not increasing T.sub.g significantly. The
resulting nanocomposite particles will then perform better as
proppants over their entire use temperature range, but without an
increase in the maximum possible use temperature itself. On the
other hand, if a suitable post-polymerization process step is
applied to the nanocomposite particles, in many cases the curing
reaction will be driven further towards completion so that T.sub.g
(and hence also the maximum possible use temperature) will increase
along with the increase induced by the nanofiller in the
compressive elastic modulus.
[0085] Processes that may be used to enhance the degree of curing
of a thermoset polymer include, but are not limited to, heat
treatment (which may be combined with stirring and/or sonication to
enhance its effectiveness), electron beam irradiation, and
ultraviolet irradiation. We focused mainly on the use of heat
treatment in order to increase the T.sub.g of the thermoset matrix
polymer, to make it possible to use nanofiller incorporation and
post-polymerization heat treatment as complementary methods, to
improve the performance characteristics of the particles even
further by combining the anticipated main benefits of each method.
FIG. 4 provides an idealized schematic illustration of the benefits
of implementing these methods and concepts.
[0086] The processes that may be used for the fabrication of the
thermoset nanocomposite particles of the invention have at least
one, and optionally two, major step(s). The required step is the
formation of said particles by means of a process that allows the
intimate embedment of the nanofiller in the polymer matrix. The
optional step is the use of an appropriate postcuring method to
advance the curing reaction of the thermoset matrix and to thus
obtain a polymer network that approaches the "fully cured" limit.
Consequently, this subsection will be further subdivided into two
subsections, dealing with polymerization and with postcure
respectively.
1. Polymerization and Network Formation in Presence of
Nanofiller
[0087] Any method for the fabrication of thermoset composite
particles known to those skilled in the art may be used to prepare
embodiments of the thermoset nanocomposite particles of the
invention. Without reducing the generality of the invention, some
such methods will be discussed below to facilitate the teaching of
the invention.
[0088] The most practical methods for the formation of composites
containing rigid thermoset matrix polymers involve the dispersion
of the filler in a liquid (aqueous or organic) medium followed by
the "in situ" formation of the crosslinked polymer network around
the filler. This is in contrast with the formation of thermoplastic
composites where melt blending can instead also be used to mix a
filler with a fully formed molten polymer. It is also in contrast
with the vulcanization of a filled rubber, where preformed polymer
chains are crosslinked in the presence of a filler.
[0089] The implementation of such methods in the preparation of
thermoset nanocomposite particles is usually more difficult to
accomplish in practice than their implementation in the preparation
of composite particles containing conventional fillers. As
discussed earlier, common challenges involve difficulties in
dispersing the nanofiller, high nanofiller dispersion viscosity,
and possible interferences of the nanofiller with polymerization
and network formation. Nonetheless, these challenges can all be
surmounted by making judicious choices of the formulation
ingredients and their proportions, and then also determining and
using the optimum processing conditions.
[0090] McDaniel, et al. (U.S. Pat. No. 6,632,527) prepared polymer
composite particles with thermoset matrix formulations. Their
formulations were based on at least one member of the group
consisting of inorganic binder, epoxy resin, novolac resin, resole
resin, polyurethane resin, alkaline phenolic resole curable with
ester, melamine resin, urea-aldehyde resin, urea-phenol-aldehyde
resin, furans, synthetic rubber, and/or polyester resin. They
taught the incorporation of conventional filler particles, whose
sizes ranged from 0.5 microns to 60 microns, at 60% to 90% by
volume. Their fabrication processes differed in details depending
on the specific formulation, but in general included steps
involving the mixing of a binder stream with a filler particle
stream, agglomerative granulation, and the curing of a granulated
material stream to obtain thermoset composite particles of the
required size and shape. These processes can also be used to
prepare the thermoset nanocomposite particles of the present
invention, where nanofillers possessing at least one principal axis
dimension shorter than 0.5 microns are used at a volume fraction
that does not exceed 60% and that is far smaller than 60% in the
currently preferred embodiments. The processes of McDaniel, et al.
(U.S. Pat. No. 6,632,527) are, hence, incorporated herein by
reference.
[0091] As was discussed earlier, many additional types of thermoset
polymers can also be used as the matrix materials in composites.
Examples include crosslinked polymers prepared from various
styrenic, acrylic or olefinic monomers (or mixtures thereof). It is
more convenient to prepare particles of such thermoset polymers (as
well as of their composites and nanocomposites) by using methods
that can produce said particles directly in the desired (usually
substantially spherical) shape during polymerization from the
starting monomers. (While it is a goal of this invention to create
spherical particles, it is understood that it is exceedingly
difficult as well as unnecessary to obtain perfectly spherical
particles. Therefore, particles with minor deviations from a
perfectly spherical shape are considered perfectly spherical for
the purposes of this disclosure.) Suspension (droplet)
polymerization is the most powerful method available for
accomplishing this objective. Two main approaches exist to
suspension polymerization. The first approach is isothermal
polymerization which is the conventional approach that has been
practiced for many decades. The second approach is "rapid rate
polymerization" as taught by Albright (U.S. Pat. No. 6,248,838)
which is incorporated herein by reference. Without reducing the
generality of the invention, suspension polymerization as performed
via the rapid rate polymerization approach taught by Albright (U.S.
Pat. No. 6,248,838) is used in the current preferred embodiments of
the invention.
2. Optional Post-Polymerization Advancement of Curing and Network
Formation
[0092] As was discussed earlier and illustrated in FIG. 1 with the
data of Bicerano, et al. (1996), typical processes for the
synthesis of thermoset polymers may result in the formation of
incompletely cured networks, and may hence produce thermosets with
lower glass transition temperatures and lower maximum use
temperatures than is achievable with the chosen formulation of
reactants. Furthermore, difficulties related to incomplete cure may
sometimes be exacerbated in thermoset nanocomposites because of the
possibility of interference by the nanofiller in polymerization and
network formation. Consequently, the use of an optional
post-polymerization process step (or a sequence of such process
steps) to advance the curing of the thermoset matrix of a particle
of the invention is an aspect of the invention. Suitable methods
include, but are not limited to, heat treatment (also known as
"annealing"), electron beam irradiation, and ultraviolet
irradiation.
[0093] Post-polymerization heat treatment is a very powerful method
for improving the properties and performance of S-DVB copolymers
(as well as of many other types of thermoset polymers) by helping
the polymer network approach its "full cure" limit. It is, in fact,
the most easily implementable method for advancing the state of
cure of S-DVB copolymer particles. However, it is important to
recognize that another post-polymerization method (such as electron
beam irradiation or ultraviolet irradiation) may be the most
readily implementable one for advancing the state of cure of some
other type of thermoset polymer. The use of any suitable method for
advancing the curing of the thermoset polymer that is being used as
the matrix of a nanocomposite of the present invention after
polymerization is within the scope of the invention.
[0094] Without reducing the generality of the invention, among the
suitable methods, heat treatment is used as the optional
post-polymerization method to enhance the curing of the thermoset
polymer matrix in the preferred embodiments of the invention. Any
desired thermal history can be optionally imposed; such as, but not
limited to, isothermal annealing at a fixed temperature;
nonisothermal heat exposure with either a continuous or a step
function temperature ramp; or any combination of continuous
temperature ramps, step function temperature ramps, and/or periods
of isothermal annealing at fixed temperatures. In practice, while
there is great flexibility in the choice of a thermal history, it
must be selected carefully to drive the curing reaction to the
maximum final extent possible without inducing unacceptable levels
of thermal degradation.
[0095] Any significant increase in T.sub.g by means of improved
curing will translate directly into an increase of comparable
magnitude in the practical softening temperature of the polymer
particles under the compressive load imposed by the subterranean
environment. Consequently, a significant increase of the maximum
possible use temperature of the thermoset polymer particles is the
most common benefit of advancing the extent of curing by heat
treatment.
[0096] A practical concern during the imposition of optional heat
treatment is related to the amount of material that is being
subjected to heat treatment simultaneously. For example, very small
amounts of material can be heat treated uniformly and effectively
in vacuum; or in any inert (non-oxidizing) gaseous medium, such as,
but not limited to, a helium or nitrogen "blanket". However, heat
transfer in a gaseous medium is not nearly as effective as heat
transfer in an appropriately selected liquid medium. Consequently,
during the optional heat treatment of large quantities of the
particles of the invention (such as, but not limited to, the output
of a run of a commercial-scale batch production reactor), it is
usually necessary to use a liquid medium, and furthermore also to
stir the particles vigorously to ensure that the heat treatment is
applied as uniformly as possible. Serious quality problems may
arise if heat treatment is not applied uniformly; for example, as a
result of the particles that were initially near the heat source
being overexposed to heat and thus damaged, while the particles
that were initially far away from the heat source are not exposed
to sufficient heat and are thus not sufficiently postcured.
[0097] If a gaseous or a liquid heat treatment medium is used, said
medium may contain, without limitation, one or a mixture of any
number of types of constituents of different molecular structure.
However, in practice, said medium must be selected carefully to
ensure that its molecules will not react with the crosslinked
polymer particles to a sufficient extent to cause significant
oxidative and/or other types of chemical degradation. In this
context, it must also be kept in mind that many types of molecules
which do not react with a polymer at ambient temperature may react
strongly with said polymer at elevated temperatures. The most
relevant example in the present context is that oxygen itself does
not react with S-DVB copolymers at room temperature, while it
causes severe oxidative degradation of S-DVB copolymers at elevated
temperatures where there would not be much thermal degradation in
its absence.
[0098] Furthermore, in considering the choice of medium for heat
treatment, it is also important to keep in mind that organic
molecules can swell organic polymers, potentially causing
"plasticization" and thus resulting in undesirable reductions of
T.sub.g and of the maximum possible use temperature. The magnitude
of any such detrimental effect increases with increasing similarity
between the chemical structures of the molecules in the heat
treatment medium and of the polymer chains. For example, a heat
transfer fluid consisting of aromatic molecules will tend to swell
a styrene-divinylbenzene copolymer particle, as well as tending to
swell a nanocomposite particle containing such a copolymer as its
matrix. The magnitude of this detrimental effect will increase with
decreasing relative amount of the crosslinking monomer
(divinylbenzene) used in the formulation. For example, a
styrene-divinylbenzene copolymer prepared from a formulation
containing only 3% by weight of divinylbenzene will be far more
susceptible to swelling in an aromatic liquid than a copolymer
prepared from a formulation containing 35% divinylbenzene.
[0099] Various means known to those skilled in the art, including
but not limited to the stirring and/or the sonication of an
assembly of particles being subjected to heat treatment, may also
be optionally used to enhance further the effectiveness of the
optional heat treatment. The rate of thermal equilibration under a
given thermal gradient, possibly combined with the application of
any such additional means, depends on many factors. These factors
include, but are not limited to, the amount of polymer particles
being heat treated simultaneously, the shapes and certain key
physical and transport properties of these particles, the shape of
the vessel being used for heat treatment, the medium being used for
heat treatment, whether external disturbances (such as stirring
and/or sonication) are being used to accelerate equilibration, and
the details of the heat exposure schedule. Simulations based on the
solution of the heat transfer equations may hence be used
optionally to optimize the heat treatment equipment and/or the heat
exposure schedule.
[0100] Without reducing the generality of the invention, in its
currently preferred embodiments, the thermoset nanocomposite
particles are left in the reactor fluid that remains after
suspension polymerization if optional heat treatment is to be used.
Said reactor fluid thus serves as the heat treatment medium; and
simulations based on the solution of the heat transfer equations
are used to optimize the heat exposure schedule. This embodiment of
the optional heat treatment works especially well (without adverse
effects such as degradation and/or swelling) in enhancing the
curing of the thermoset matrix polymer in the currently preferred
compositions of matter of the invention. Said preferred
compositions of matter consist of terpolymers of styrene,
ethylvinylbenzene and divinylbenzene. Since the reactor fluid that
remains after the completion of suspension polymerization is
aqueous while these terpolymers are very hydrophobic, the reactor
fluid serves as an excellent heat transfer medium which does not
swell the particles. The use of the reactor fluid as the medium for
the optional heat treatment also has the advantage of simplicity
since the particles would have needed to be removed from the
reactor fluid and placed in another fluid as an extra step before
heat treatment if an alternative fluid had been required.
[0101] It is, however, important to reemphasize the much broader
scope of the invention and the fact that the particular currently
preferred embodiments summarized above constitute just a few among
the vast variety of possible qualitatively different classes of
embodiments. For example, if a hydrophilic thermoset polymer
particle were to be developed as an alternative preferred
embodiment of the invention in future work, it would obviously not
be possible to subject such an embodiment to heat treatment in an
aqueous slurry, and a hydrophobic heat transfer fluid would work
better for its optional heat treatment.
C. Applications
[0102] The obvious practical advantages [see a review by Edgeman
(2004)] of developing the ability to use lightweight particles that
possess almost neutral buoyancy relative to water have stimulated a
considerable amount of work over the years. However, progress in
this field of invention has been very slow as a result of the many
technical challenges that exist to the successful development of
cost-effective lightweight particles that possess sufficient
stiffness, strength and heat resistance. The present invention has
resulted in the development of such stiff, strong, tough, heat
resistant, and environmentally resistant ultralightweight
particles; and also of cost-effective processes for the fabrication
of said particles. As a result, a broad range of potential
applications can be envisioned and are being pursued for the use of
the thermoset polymer nanocomposite particles of the invention in
the construction, drilling, completion and/or fracture stimulation
of oil and natural gas wells. Without reducing the generality of
the invention, in its currently preferred embodiments, the specific
applications that are already being evaluated are as a proppant
partial monolayer, a proppant pack, an integral component of a
gravel pack completion, a ball bearing, a solid lubricant, a
drilling mud constituent, and/or a cement additive.
[0103] It is also important to note that the current selection of
preferred embodiments of the invention has resulted from our focus
on application opportunities in the construction, drilling,
completion and/or fracture stimulation of oil and natural gas
wells. Many other applications can also be envisioned for the
compositions of matter that fall within the scope of thermoset
nanocomposite particles of the invention. For example, one such
application is described by Nishimori, et. al. (JP1992-22230), who
developed heat-treated S-DVB copolymer (but not composite)
particles prepared from formulations containing very high DVB
weight fractions for use in liquid crystal display panels.
Alternative embodiments of the thermoset copolymer nanocomposite
particles of the present invention, tailored towards the
performance needs of that application and benefiting from its less
restrictive cost limitations, could potentially also be used in
liquid crystal display panels. Considered from this perspective, it
can be seen readily that the potential applications of the
particles of the invention extend far beyond their uses by the oil
and natural gas industry.
BRIEF DESCRIPTION OF THE DRAWINGS
[0104] The accompanying drawings, which are included to provide
further understanding of the invention and are incorporated in and
constitute a part of this specification, illustrate embodiments of
the invention and, together with the description, serve to explain
the principles of the invention.
[0105] FIG. 1 shows the effects of advancing the curing reaction in
a series of isothermally polymerized styrene-divinylbenzene (S-DVB)
copolymers containing different DVB weight fractions via heat
treatment. The results of scans of S-DVB beads containing various
weight fractions of DVB (w.sub.DVB), obtained by Differential
Scanning calorimetry (DSC), and reported by Bicerano, et al.
(1996), are compared. It is seen that the T.sub.g of typical
"as-polymerized" S-DVB copolymers, as measured by the first DSC
scan, increased only slowly with increasing w.sub.DVB, and
furthermore that the rate of further increase of T.sub.g slowed
down drastically for w.sub.DVB>0.08. By contrast, in the second
DSC scan (performed on S-DVB specimens whose curing had been driven
much closer to completion as a result of the temperature ramp that
had been applied during the first scan), T.sub.g grew much more
rapidly with w.sub.DVB over the entire range of up to
w.sub.DVB=0.2458 that was studied.
[0106] FIG. 2 provides an idealized, generic and schematic
two-dimensional illustration of how a very small volume fraction of
a nanofiller may be able to "span" and thus "bridge through" a vast
amount of space, thus potentially enhancing the load bearing
ability of the matrix polymer significantly at much smaller volume
fractions than possible with conventional fillers.
[0107] FIG. 3 illustrates the "aggregates" in which the "primary
particles" of nanofillers such as nanoscale carbon black, fumed
silica and fumed alumina commonly occur. Such aggregates may
contain many very small primary particles, often arranged in a
"fractal" pattern, resulting in aggregate principal axis dimensions
that are also shorter than 0.5 microns. These aggregates (and not
the individual primary particles that constitute them) are,
usually, the smallest units of such nanofillers that are dispersed
in a polymer matrix under normal fabrication conditions, when the
forces holding the aggregates together in the much larger
"agglomerates" are overcome successfully. This illustration was
reproduced from the product literature of Cabot Corporation.
[0108] FIG. 4 provides an idealized schematic illustration, in the
context of the resistance of thermoset polymer particles to
compression as a function of the temperature, of the most common
benefits of using the methods of the present invention. In most
cases, the densification of the crosslinked polymer network via
post-polymerization heat treatment will have the main benefit of
increasing the softening (and hence also the maximum possible use)
temperature, along with improving the environmental resistance. On
the other hand, in most cases, nanofiller incorporation will have
the main benefits of increasing the stiffness and strength. The use
of nanofiller incorporation and post-polymerization heat treatment
together, as complementary methods, will thus often be able to
provide all (or at least most) of these benefits
simultaneously.
[0109] FIG. 5 provides a process flow diagram depicting the
preparation of the example. It contains four major blocks;
depicting the preparation of the aqueous phase (Block A), the
preparation of the organic phase (Block B), the mixing of these two
phases followed by suspension polymerization (Block C), and the
further process steps used after polymerization to obtain the
"as-polymerized" and "heat-treated" samples of particles (Block
D).
[0110] FIG. 6 shows the variation of the temperature with time
during polymerization.
[0111] FIG. 7 shows the results of the measurement of the glass
transition temperatures (T.sub.g) of the three heat-treated
thermoset nanocomposite samples via differential scanning
calorimetry (DSC). The samples have identical compositions. They
differ only as a result of the use of different heat treatment
conditions after polymerization. T.sub.g was defined as the
temperature at which the curve showing the heat flow as a function
of the temperature goes through its inflection point.
[0112] FIG. 8 provides a schematic illustration of the
configuration of the conductivity cell.
[0113] FIG. 9 shows the measured liquid conductivity of a packing
of particles of 14/16 U.S. mesh size (diameters ranging from 1.19
mm to 1.41 mm) from Sample 40m200C, at a coverage of 0.02
lb/ft.sup.2, under a closure stress of 4000 psi at a temperature of
190.degree. F., as a function of time.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0114] Because the invention will be understood better after
further discussion of its currently preferred embodiments, further
discussion of said embodiments will now be provided. It is
understood that said discussion is being provided without reducing
the generality of the invention, since persons skilled in the art
can readily imagine many additional embodiments that fall within
the full scope of the invention as taught in the SUMMARY OF THE
INVENTION section.
A. Nature, Attributes and Applications of Currently Preferred
Embodiments
[0115] The currently preferred embodiments of the invention are
lightweight thermoset nanocomposite particles possessing high
stiffness, strength, temperature resistance, and resistance to
aggressive environments. These attributes, occurring in
combination, make said particles especially suitable for use in
many challenging applications in the construction, drilling,
completion and/or fracture stimulation of oil and natural gas
wells. Said applications include the use of said particles as a
proppant partial monolayer, a proppant pack, an integral component
of a gravel pack completion, a ball bearing, a solid lubricant, a
drilling mud constituent, and/or a cement additive.
B. Thermoset Polymer Matrix
[0116] 1. Constituents
[0117] The thermoset matrix in said particles consists of a
terpolymer of styrene (S, non-crosslinking), ethyvinylbenzene (EVB,
also non-crosslinking), and divinylbenzene (DVB, crosslinking). The
preference for such a terpolymer instead of a copolymer of S and
DVB is a result of economic considerations. To summarize, DVB comes
mixed with EVB in the standard product grades of DVB, and the cost
of DVB increases rapidly with increasing purity in special grades
of DVB. EVB is a non-crosslinking (difunctional) styrenic monomer.
Its incorporation into the thermoset matrix does not result in any
significant changes in the properties of the thermoset matrix or of
nanocomposites containing said matrix, compared with the use of S
as the sole non-crosslinking monomer. Consequently, it is far more
cost-effective to use a standard (rather than purified) grade of
DVB, thus resulting in a terpolymer where some of the repeat units
originate from EVB.
[0118] 2. Proportions
[0119] The amount of DVB in said terpolymer ranges from 3% to 35%
by weight of the starting mixture of the three reactive monomers
(S, EVB and DVB) because different applications require different
maximum possible use temperatures. Even when purchased in standard
product grades where it is mixed with a large weight fraction of
EVB, DVB is more expensive than S. It is, hence, useful to develop
different product grades where the maximum possible use temperature
increases with increasing weight fraction of DVB. Customers can
then purchase the grades of said particles that meet their specific
application needs as cost-effectively as possible.
C. Nanofiller
[0120] 1. Constituents
[0121] The Monarch.TM. 280 product grade of nanoscale carbon black
supplied by Cabot Corporation is being used as the nanofiller in
said particles. The reason is that it has a relatively low specific
surface area, high structure, and a "fluffy" product form;
rendering it especially easy to disperse.
[0122] 2. Proportions
[0123] The use of too low a volume fraction of carbon black results
in ineffective reinforcement. The use of too high a volume fraction
of carbon black may result in difficulties in dispersing the
nanofiller, dispersion viscosities that are too high to allow
further processing with available equipment, and detrimental
interference in polymerization and network formation. The amount of
carbon black ranges from 0.1% to 15% by volume of said particles
because different applications require different levels of
reinforcement. Carbon black is more expensive than the monomers (S,
EVB and DVB) currently being used in the synthesis of the thermoset
matrix. It is, therefore, useful to develop different product
grades where the extent of reinforcement increases with increasing
volume fraction of carbon black. Customers can then purchase the
grades of said particles that meet their specific application needs
as cost-effectively as possible.
D. Polymerization
[0124] Suspension polymerization is performed via rapid rate
polymerization, as taught by Albright (U.S. Pat. No. 6,248,838)
which is incorporated herein by reference, for the fabrication of
said particles. Rapid rate polymerization has the advantage,
relative to conventional isothermal polymerization, of producing
more physical entanglements in thermoset polymers (in addition to
the covalent crosslinks). Suspension polymerization involves the
preparation of an the aqueous phase and an organic phase prior to
the commencement of the polymerization process. The Monarch.TM. 280
carbon black particles are dispersed in the organic phase prior to
polymerization. The most important additional formulation component
(besides the reactive monomers and the nanofiller particles) that
is used during polymerization is the initiator. The initiator may
consist of one type molecule or a mixture of two or more types of
molecules that have the ability to function as initiators.
Additional formulation components, such as catalysts, inhibitors,
dispersants, stabilizers, rheology modifiers, buffers,
antioxidants, defoamers, impact modifiers, plasticizers, pigments,
flame retardants, smoke retardants, or mixtures thereof, may also
be used when needed. Some of the additional formulation
component(s) may become either partially or completely incorporated
into the particles in some embodiments of the invention.
E. Attainable Particle Sizes
[0125] Suspension polymerization produces substantially spherical
polymer particles. (While it is a goal of this invention to create
spherical particles, it is understood that it is exceedingly
difficult as well as unnecessary to obtain perfectly spherical
particles. Therefore, particles with minor deviations from a
perfectly spherical shape are considered perfectly spherical for
the purposes of this disclosure.) Said particles can be varied in
size by means of a number of mechanical and/or chemical methods
that are well-known and well-practiced in the art of suspension
polymerization. Particle diameters attainable by such means range
from submicron values up to several millimeters. Hence said
particles may be selectively manufactured over the entire range of
sizes that are of present interest and/or that may be of future
interest for applications in the oil and natural gas industry.
F. Optional Further Selection of Particles by Size
[0126] Optionally, after the completion of suspension
polymerization, said particles can be separated into fractions
having narrower diameter ranges by means of methods (such as, but
not limited to, sieving techniques) that are well-known and
well-practiced in the art of particle separations. Said narrower
diameter ranges include, but are not limited to, nearly
monodisperse distributions. Optionally, assemblies of particles
possessing bimodal or other types of special distributions, as well
as assemblies of particles whose diameter distributions follow
statistical distributions such as gaussian or log-normal, can also
be prepared.
[0127] The optional preparation of assemblies of particles having
diameter distributions of interest from any given "as polymerized"
assembly of particles can be performed before or after any optional
heat treatment of said particles. Without reducing the generality
of the invention, in the currently most preferred embodiments of
the invention, any optional preparation of assemblies of particles
having diameter distributions of interest from the product of a run
of the pilot plant or production plant reactor is performed after
the completion of any optional heat treatment of said
particles.
[0128] The particle diameters of current practical interest for
various uses in the construction, drilling, completion and/or
fracture stimulation of oil and natural gas wells range from 0.1 to
4 millimeters. The specific diameter distribution that would be
most effective under given circumstances depends on the details of
the subterranean environment in addition to depending on the type
of application. The diameter distribution that would be most
effective under given circumstances may be narrow or broad,
monomodal or bimodal, and may also have other special features
(such as following a certain statistical distribution function)
depending on both the details of the subterranean environment and
the type of application.
G. Optional Heat Treatment
[0129] Said particles are left in the reactor fluid that remains
after suspension polymerization if optional heat treatment is to be
used. Said reactor fluid thus serves as the heat treatment medium.
This approach works especially well (without adverse effects such
as degradation and/or swelling) in enhancing the curing of said
particles where the polymer matrix consists of a terpolymer of S,
EVB and DVB. Since the reactor fluid that remains after the
completion of suspension polymerization is aqueous while these
terpolymers are very hydrophobic, the reactor fluid serves as an
excellent heat transfer medium which does not swell the particles.
The use of the reactor fluid as the medium for the optional heat
treatment also has the advantage of simplicity since the particles
would have needed to be removed from the reactor fluid and placed
in another fluid as an extra step before heat treatment if an
alternative fluid had been required. Detailed and realistic
simulations based on the solution of the heat transfer equations
are often used optionally to optimize the heat exposure schedule if
optional heat treatment is to be used. It has been found that such
simulations become increasingly useful with increasing quantity of
particles that will be heat treated simultaneously. The reason is
the finite rate of heat transfer. Said finite rate results in
slower and more difficult equilibration with increasing quantity of
particles and hence makes it especially important to be able to
predict how to cure most of the particles further uniformly and
sufficiently without overexposing many of the particles to
heat.
EXAMPLE
[0130] The currently preferred embodiments of the invention will be
understood better in the context of a specific example. It is to be
understood that said example is being provided without reducing the
generality of the invention. Persons skilled in the art can readily
imagine many additional examples that fall within the scope of the
currently preferred embodiments as taught in the DETAILED
DESCRIPTION OF THE INVENTION section. Persons skilled in the art
can, furthermore, also readily imagine many alternative embodiments
that fall within the full scope of the invention as taught in the
SUMMARY OF THE INVENTION section.
A. SUMMARY
[0131] The thermoset matrix was prepared from a formulation
containing 10% DVB by weight of the starting monomer mixture. The
DVB had been purchased as a mixture where only 63% by weight
consisted of DVB. The actual polymerizable monomer mixture used in
preparing the thermoset matrix consisted of roughly 84.365% S,
5.635% EVB and 10% DVB by weight.
[0132] Carbon black (Monarch 280) was incorporated into the
particles, at 0.5% by weight, via dispersion in the organic phase
of the formulation prior to polymerization. Since the specific
gravity of carbon black is roughly 1.8 while the specific gravity
of the polymer is roughly 1.04, the amount of carbon black
incorporated into the particles was roughly 0.29% by volume.
[0133] Suspension polymerization was performed in a pilot plant
reactor, via rapid rate polymerization as taught by Albright (U.S.
Pat. No. 6,248,838) which is incorporated herein by reference. In
applying this method, the "dual initiator" approach, wherein two
initiators with different thermal stabilities are used to help
drive the reaction of DVB further towards completion, was
utilized.
[0134] The required tests only require a small quantity of
particles. The use of a liquid medium (such as the reactor fluid)
is unnecessary for the heat treatment of a small sample. Roughly
500 grams of particles were hence removed from the slurry, washed,
spread very thin on a tray, heat-treated for ten minutes at
200.degree. C. in an oven in an inert gas environment, and
submitted for testing.
[0135] The glass transition temperature of these "heat-treated"
particles, and the liquid conductivity of packings thereof, were
then measured by independent testing laboratories (Impact
Analytical in Midland, Mich., and FracTech Laboratories in Surrey,
United Kingdom, respectively).
[0136] FIG. 5 provides a process flow diagram depicting the
preparation of the example. It contains four major blocks;
depicting the preparation of the aqueous phase (Block A), the
preparation of the organic phase (Block B), the mixing of these two
phases followed by suspension polymerization (Block C), and the
further process steps used after polymerization to obtain the
"as-polymerized" and "heat-treated" samples of particles (Block
D).
[0137] The following subsections will provide further details on
the formulation, preparation and testing of this working example,
to enable persons who are skilled in the art to reproduce the
example.
B. FORMULATION
[0138] An aqueous phase and an organic phase must be prepared prior
to suspension polymerization. The aqueous phase and the organic
phase, which were prepared in separate beakers and then used in the
suspension polymerization of the particles of this example, are
described below.
[0139] 1. Aqueous Phase
[0140] The aqueous phase used in the suspension polymerization of
the particles of this example, as well as the procedure used to
prepare said aqueous phase, are summarized in TABLE 1.
TABLE-US-00001 TABLE 1 The aqueous phase was prepared by adding
Natrosol Plus 330 and gelatin (Bloom strength 250) to water,
heating to 65.degree. C. to disperse the Natrosol Plus 330 and the
gelatin in the water, and then adding sodium nitrite and sodium
carbonate. Its composition is listed below. INGREDIENT WEIGHT (g) %
Water 1493.04 98.55 Natrosol Plus 330 (hydroxyethylcellulose) 7.03
0.46 Gelatin (Bloom strength 250) 3.51 0.23 Sodium Nitrite
(NaNO.sub.2) 4.39 0.29 Sodium Carbonate (Na.sub.2CO.sub.3) 7.03
0.46 Total Weight in Grams 1515.00 100.00
[0141] 2. Organic Phase
[0142] The organic phase used in the suspension polymerization of
the particles of this example, as well as the procedure used to
prepare said organic phase, are summarized in TABLE 2. Note that
the nanofiller (carbon black) was added to the organic phase in
this particular example.
TABLE-US-00002 TABLE 2 The organic phase was prepared by placing
the monomers, benzoyl peroxide (an initiator), t-amyl
peroxy(2-ethylhexyl)monocarbonate (TAEC, also an initiator),
Disperbyk-161 and carbon black together and agitating the resulting
mixture for at least 15 minutes to disperse carbon black in the
mixture. Its composition is listed below. After taking the other
components of the 63% DVB mixture into account, the polymerizable
monomer mixture actually consisted of roughly 84.365% S, 5.635% EVB
and 10% DVB by weight. The total polymerizable monomer weight of
was 1356.7 grams. The resulting thermoset nanocomposite particles
thus contained [100 .times. 6.8/(1356.7 + 6.8)] = 0.5% by weight of
carbon black. INGREDIENT WEIGHT (g) % Styrene (pure) 1144.58 82.67
Divinylbenzene (63% DVB, 215.35 15.56 98.5% polymerizable monomers)
Carbon black (Monarch 280) 6.8 0.49 Benzoyl peroxide 13.567 0.98
t-Amyl peroxy(2-ethylhexyl)monocarbonate 4.07 0.29 (TAEC)
Disperbyk-161 0.068 0.0049 Total Weight in Grams 1384.435 100
C. PREPARATION OF PARTICLES FROM FORMULATION
[0143] Once the formulation is prepared, its aqueous and organic
phases are mixed, polymerization is performed, and "as-polymerized"
and "heat-treated" particles are obtained, as described below.
[0144] 1. Mixing
[0145] The aqueous phase was added to the reactor at 65.degree. C.
The organic phase was then introduced over roughly 5 minutes with
agitation at the rate of 90 rpm. The mixture was held at 65.degree.
C. with stirring at the rate of 90 rpm for at least 15 minutes or
until proper dispersion had taken place as manifested by the
equilibration of the droplet size distribution.
[0146] 2. Polymerization
[0147] The temperature was ramped from 65.degree. C. to 78.degree.
C. in 10 minutes. It was then further ramped from 78.degree. C. to
90.degree. C. at the rate of 0.1.degree. C. per minute in 120
minutes. It was then held at 90.degree. C. for 90 minutes to
provide most of the conversion of monomer to polymer, with benzoyl
peroxide (half life of one hour at 92.degree. C.) as the effective
initiator. It was then further ramped to 115.degree. C. in 30
minutes and held at 115.degree. C. for 180 minutes to advance the
curing with TAEC (half life of one hour at 117.degree. C.) as the
effective initiator. The particles were thus obtained in an aqueous
slurry. FIG. 6 shows the variation of the temperature with time
during polymerization.
[0148] 3. "As-Polymerized" Particles
[0149] The aqueous slurry was cooled to 40.degree. C. It was then
poured onto a 60 mesh (250 micron) sieve to remove the aqueous
reactor fluid as well as any undesirable small particles that may
have formed during polymerization. The "as-polymerized" beads of
larger than 250 micron diameter obtained in this manner were then
washed three times with warm (40.degree. C. to 50.degree. C.)
water
[0150] 4. "Heat-Treated" Particles
[0151] Three sets of "heat-treated" particles, which were imposed
to different thermal histories during the post-polymerization heat
treatment, were prepared from the "as-polymerized" particles. In
preparing each of these heat-treated samples, washed beads were
removed from the 60 mesh sieve, spread very thin on a tray, placed
in an oven under an inert gas (nitrogen) blanket, and subjected to
the desired heat exposure. Sample 10m200C was prepared with
isothermal annealing for 10 minutes at 200.degree. C. Sample
40m200C was prepared with isothermal annealing for 40 minutes at
200.degree. C. to explore the effects of extending the duration of
isothermal annealing at 200.degree. C. Sample 10m220C was prepared
with isothermal annealing for 10 minutes at 220.degree. C. to
explore the effects of increasing the temperature at which
isothermal annealing is performed for a duration of 10 minutes. In
each case, the oven was heated to 100.degree. C., the sample was
placed in the oven and covered with a nitrogen blanket; and the
temperature was then increased to its target value at a rate of
2.degree. C. per minute, held at the target temperature for the
desired length of time, and finally allowed to cool to room
temperature by turning off the heat in the oven. Some particles
from each sample were sent to Impact Analytical for the measurement
of T.sub.g via DSC.
[0152] Particles of 14/16 U.S. mesh size were isolated from Sample
40m200C by some additional sieving. This is a very narrow size
distribution, with the particle diameters ranging from 1.19 mm to
1.41 mm. This nearly monodisperse assembly of particles was sent to
FracTech Laboratories for the measurement of the liquid
conductivity of its packings.
D. REFERENCE SAMPLE
[0153] A Reference Sample was also prepared, to provide a baseline
against which the data obtained for the particles of the invention
can be compared.
[0154] The formulation and the fabrication process conditions used
in the preparation of the Reference Sample differed from those used
in the preparation of the examples of the particles of the
invention in two key aspects. Firstly, carbon black was not used in
the preparation of the Reference Sample. Secondly,
post-polymerization heat treatment was not performed in the
preparation of the Reference Sample. Consequently, while the
examples of the particles of the invention consisted of a
heat-treated and carbon black reinforced thermoset nanocomposite,
the particles of the Reference Sample consisted of an unfilled and
as-polymerized thermoset polymer that has the same composition as
the thermoset matrix of the particles of the invention.
[0155] Some particles from the Reference Sample were sent to Impact
Analytical for the measurement of T.sub.g via DSC. In addition,
particles of 14/16 U.S. mesh size were isolated from the Reference
Sample by sieving and sent to FracTech Laboratories for the
measurement of the liquid conductivity of their packings.
E. DIFFERENTIAL SCANNING CALORIMETRY
[0156] DSC experiments (ASTM E1356-03) were carried out by using a
TA Instruments Q100 DSC with nitrogen flow of 50 mL/min through the
sample compartment. Roughly nine milligrams of each sample were
weighed into an aluminum sample pan, the lid was crimped onto the
pan, and the sample was then placed in the DSC instrument. The
sample was then scanned from 5.degree. C. to 225.degree. C. at a
rate of 10.degree. C. per minute. The instrument calibration was
checked with NIST SRM 2232 indium. Data analysis was performed by
using the TA Universal Analysis V4.1 software.
[0157] DSC data for the heat-treated samples are shown in FIG. 7.
T.sub.g was defined as the temperature at which the curve for the
heat flow as a function of the temperature went through its
inflection point. The results are summarized in TABLE 3. It is seen
that the extent of polymer curing in Sample 10m220C is comparable
to that in Sample 40m200C, and that the extent of polymer curing in
both of these samples has advanced significantly further than that
in Sample 10m200C whose T.sub.g was only slightly higher than that
of the Reference Sample.
TABLE-US-00003 TABLE 3 Glass transitions temperatures (T.sub.g) of
the three heat-treated samples and of the Reference Sample, in
.degree. C. In addition to being an "as-polymerized" (rather than a
heat-treated) sample, the Reference Sample also differs from the
other three samples since it is an unfilled sample while the other
three samples each contain 0.5% by weight carbon black. ISOTHERMAL
HEAT T.sub.g SAMPLE TREATMENT IN NITROGEN (.degree. C.) Reference
Sample None 117.17 10m200C For 10 minutes at a temperature of
200.degree. C. 122.18 10m220C For 10 minutes at a temperature of
220.degree. C. 131.13 40m200C For 40 minutes at a temperature of
200.degree. C. 131.41
F. LIQUID CONDUCTIVITY MEASUREMENT
[0158] A fracture conductivity cell allows a particle packing to be
subjected to desired combinations of compressive stress (simulating
the closure stress on a fracture in a downhole environment) and
elevated temperature over extended durations, while the flow of a
fluid through the packing is measured. The flow capacity can be
determined from differential pressure measurements. The
experimental setup is illustrated in FIG. 8.
[0159] Ohio sandstone, which has roughly a compressive elastic
modulus of 4 Mpsi and a permeability of 0.1 mD, was used as a
representative type of outcrop rock. Wafers of thickness 9.5 mm
were machined to 0.05 mm precision and one rock was placed in the
cell. The sample was split to ensure that a representative sample
is achieved in terms of its particle size distribution and then
weighed. The particles were placed in the cell and leveled. The top
rock was then inserted. Heated steel platens were used to provide
the correct temperature simulation for the test. A thermocouple
inserted in the middle port of the cell wall recorded the
temperature of the pack. A servo-controlled loading ram provided
the closure stress. The conductivity of deoxygenated
silica-saturated 2% potassium chloride (KCl) brine of pH 7 through
the pack was measured.
[0160] The conductivity measurements were performed by using the
following procedure: [0161] 1. A 70 mbar full range differential
pressure transducer was activated by closing the bypass valve and
opening the low pressure line valve. [0162] 2. When the
differential pressure appeared to be stable, a tared volumetric
cylinder was placed at the outlet and a stopwatch was started.
[0163] 3. The output of the differential pressure transducer was
fed to a data logger 5-digit resolution multimeter which logs the
output every second during the measurement. [0164] 4. Fluid was
collected for 5 to 10 minutes, after which time the flow rate was
determined by weighing the collected effluent. The mean value of
the differential pressure was retrieved from the multimeter
together with the peak high and low values. If the difference
between the high and low values was greater than the 5% of the
mean, the data point was disregarded. [0165] 5. The temperature was
recorded from the inline thermocouple at the start and at the end
of the flow test period. If the temperature variation was greater
than 0.5.degree. C., the test was disregarded. The viscosity of the
fluid was obtained from the measured temperature by using viscosity
tables. No pressure correction is made for brine at 100 psi. The
density of brine at elevated temperature was obtained from these
tables. [0166] 6. At least three permeability determinations were
made at each stage. The standard deviation of the determined
permeabilities was required to be less than 1% of the mean value
for the test sequence to be considered acceptable. [0167] 7. At the
end of the permeability testing, the widths of each of the four
corners of the cell were determined to 0.01 mm resolution by using
vernier calipers. The test results are summarized in TABLE 4.
TABLE-US-00004 [0167] TABLE 4 Measurements on packings of 14/16
U.S. mesh size of Sample 40m200C and of the Reference Sample at a
coverage of 0.02 lb/ft.sup.2. The conductivity (mDft) of
deoxygenated silica saturated 2% potassium chloride (KCl) brine of
pH 7 through each sample was measured at a temperature of
190.degree. F. (87.8.degree. C.) under a compressive stress of 4000
psi (27.579 MPa). Time Reference Sample Time Sample 40m200C (hours)
Conductivity (mDft) (hours) Conductivity (mDft) 27 1179 45 1329 49
1040 85 1259 72 977 109 1219 97 903 133 1199 120 820 157 1172 145
772 181 1151 168 736 205 1126 192 728 233 1110 218 715 260 720
[0168] These results are shown in FIG. 9. They demonstrate clearly
the advantage of the particles of the invention in terms of the
enhanced retention of liquid conductivity under a compressive
stress of 4000 psi at a temperature of 190.degree. F.
* * * * *